Multi-element resistive memory

ABSTRACT

A memory device, and methods relating thereto, having memory cells in which a single  an access transistor controls the grounding of at least two storage  resistive memory elements, such as resistive storage elements,  for purposes of reading the respective logical states of the storage elements. The logical states of the storage elements are decoupled from one another and are read independently. The storage elements are disposed in respective layers. Each storage element is coupled to first and second conductors having  for reading the memory that have respective, parallel, longitudinal axes. The longitudinal axes are oriented substantially parallel to one another, at least in proximity to a particular storage element.

FIELD OF INVENTION

The present invention relates to the field of resistive memory, twoexamples of which are a Magnetic Random Access Memory (MRAM) and aProgrammable Conductive Random Access Memory (PCRAM). More particularly,it relates to a transistor-switched resistive memory architecture.

BACKGROUND OF THE INVENTION

Resistive memory integrated circuits represent data using an electricalresistance of a resistive memory element. One group of resistive memorytechnologies is Magnetic Random Access Memory (MRAM) technology. Anotheris Programmable Conductive Random Access Memory (PCRAM). Theseresistance memory technologies have the ability to provide non-volatileor semi-volatile random access memory devices which require no, orinfrequent, refreshing.

MRAM technology operates by sensing the electrical resistance of amagneto-resistive memory element, where the resistance depends on amagnetization state of the memory element. When the resistive memoryelement is magnetized with a field oriented in a first direction, itrepresents a first stored logical state. When the element is magnetizedwith a field oriented in a second direction, it represents a second,different, logical state. The orientation of the magnetic field of thememory cell is altered by passing electrical currents through one ormore conductors disposed in proximity to the magneto-resistive memoryelement.

It is known, for example, to use Magnetic Tunnel Junction (MTJ) devicesas magneto-resistive memory elements. The resistance of an MJT devicedepends on the level of quantum tunneling that occurs across a thindielectric film interposed between two magnetic electrical conductors.One of the magnetic electrical conductors is referred to as a “pinnedlayer” and has a relatively high magnetic coercivity. The pinned layerhas a magnetic field direction fixed in a first orientation. The othermagnetic electrical conductor is referred to as a “sense layer” (or“programmed layer”). The magnetic coercivity of the sense layer is low,as compared with that of the pinned layer, and the sense layer issubject to magnetization and re-magnetization to change the orientationof its magnetic field direction through during operation of the MRAMdevice.

When the sense layer is magnetized to have a magnetic field directionparallel to that of the pinned layer (the “easy” direction), theelectrical resistance of the device has a first value. When the senselayer is magnetized to have a magnetic field direction anti-parallel tothat of the pinned layer (the “hard” direction), the electricalresistance of the device has a second value. The two values ofelectrical resistance are used to represent two binary values, and thusstore a binary digit (bit) of data. Toggling the sense layermagnetization between the easy and hard directions represents togglingbetween bit states.

A typical MRAM device includes many memory elements along with bit andword lines and addressing and driving circuitry. Some MRAM devicesinclude access transistors adapted to disconnect each memory cell fromthe word and/or bit lines except when the particular memory cell isbeing read. This architecture produces reliable and fast data access atthe expense of reduced storage density. In an alternative “crosspoint”architecture, MRAM memory elements are directly connected between wordand bit lines, without access transistors. This approach increases datadensity at the expense of relatively more difficult data state sensingoperations and consequently slower data access.

It is desirable to have a resistive memory device with both high accessspeed and high storage density.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a resistive memory device with bothhigh access speed and high storage density. The embodiments includephysically layered resistive memory elements and respective controlledaccess transistors. In one exemplary embodiment, the invention includesaccess transistors in a NOR-structured architecture, with eachtransistor coupled to a plurality of layered resistive memory devices.In this embodiment, the inclusion of an access transistor in each memorycell allows fast read operations with relatively simple read circuitry.Because the MRAM storage elements are layered, one upon another, highstorage density is achieved.

Embodiments of the invention also include a plurality of multi-bit MRAMcells disposed in an array. Each MRAM cell includes at least one accesstransistor. The access transistor is a field effect transistor with asource coupled to a device ground and a gate coupled to a word-line.Each access transistor has a drain mutually coupled to respective pinnedlayers of at least first and second MRAM resistive elements. Eachresistive element includes, in addition to a pinned layer, a sense layerand an insulating layer. The sense and pinned layers of the first andsecond resistive elements of each cell are disposed in substantiallyparallel spaced relation to one another, and the two resistive elementsare stacked such that the two resistive elements are disposed one aboveanother and above the access transistor of the cell. The sense layer ofeach resistive element is coupled to a read/write conductor of thearray. The resulting arrangement includes resistive storage elementsdisposed in two or more two-dimensional layers to form athree-dimensional array of resistive storage elements.

Embodiments of the invention also include a plurality of switchingdevices (transistors) disposed in a two-dimensional array of transistorssuch that the two-dimensional array of transistors is disposed along aface of the three-dimensional array of resistive storage elements. Eachtransistor is coupled to, and controls, the resistive elements disposedabove it in the three-dimensional array. In one aspect of the invention,the transistors, such as wired-NOR transistors, are formed according toconventional FLASH-memory technology. This presents the advantage ofemploying previously developed equipment and procedures which mayconsequently be available at reduced cost.

In other embodiments of the invention PCRAM memory elements are employedin place of the MRAM resistance elements described above. In PCRAM, thememory element includes a resistance variable or capacitance variablematerial, such as a chalcogenic material, disposed between twoelectrodes. An electrical potential impressed across the two electrodescan cause the resistance variable or capacitance variable material tochange state in a detectable fashion. For example, the resistance orcapacitance between the electrodes may be varied. Further description ofPCRAM memory elements is found in U.S. patent application PublicationNo. U.S. 2003/0032254-A1 to Gilton (Feb. 13, 2003), U.S. patentapplication Publication No. U.S. 2003/00405049-A1 to Campbell et al.(Mar. 6, 2003), U.S. patent application Publication No. U.S.2003/0001229-A1 to Moore et al. (Jan. 2, 2003), U.S. patent applicationPublication No. U.S. 2003/0123248-A1 to Moore et al. (Sep. 5, 2002),U.S. patent application Publication No. U.S. 2003/0123170-A1 to Moore etal. (Sep. 5, 2002), which publications are herewith incorporated intheir entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments of the invention given below with reference to theaccompanying drawings, in which:

FIG. 1A shows a sectional side view of an MRAM cell according to oneembodiment of the invention;

FIG. 1B shows a sectional side view of an MRAM cell according to oneembodiment of the invention, including grounding through a groundedwell;

FIG. 2 shows an MRAM memory storage element including five layers ofmaterial;

FIG. 3A shows a dual transistor structure;

FIG. 3B shows a FLASH memory architecture;

FIGS. 4A and 4B are illustrations showing top views of a portion of anMRAM cell according to different embodiments of the invention;

FIG. 5 shows a spatial relationship between memory cells according toone embodiment of the invention;

FIG. 6 shows, in electrical schematic form, an MRAM cell according toone aspect of the invention;

FIG. 7 shows a portion of an MRAM device illustrating electricalcurrents according to one aspect of the invention;

FIG. 8 shows a portion of an MRAM device according to one aspect of theinvention;

FIG. 9 shows a portion of an MRAM device according to one aspect of theinvention; and

FIG. 10 shows a block diagram of a digital system incorporating an MRAMmemory device according to one aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention embodiments described herein are described in relation toan MRAM resistive memory cell. However, it should be understood that theinvention has more general applicability to any resistive memory cell,such as in, for example, a programmable conductive Random Access Memory(PCRAM). FIG. 1A shows an embodiment of a resistive memory cell 10 e.g.,an MRAM cell, according to one embodiment of the invention. The cell 10includes an upper memory portion 120 and a lower memory portion 160.Each memory portion 120, 160 has a conventional MRAM storage element100, 200. As shown in FIG. 2, the exemplary MRAM storage elements 100,200 each include five layers of material in a stacked arrangement. Thelayers include a seed layer 102, pinning layer 104, pinned layer 106,dielectric layer 108, and a sense layer 110. As discussed above, thepinned layer 106 exhibits a magnetic field in an orientation that ispermanently set during manufacturing. Other layers and arrangements maybe used for MRAM storage elements 100, 200, or for other types ofresistive memory.

Referring again to FIG. 1A, the first MRAM storage element 100 iscoupled at an upper surface to a first read/write conductor 126 and at alower surface to a first read conductor 128. The first read/rightread/write conductor 126 has a longitudinal axis 127 in the vicinity ofthe MRAM storage element 100. The first read conductor 128 also has alongitudinal axis 129. According to one embodiment of the invention,longitudinal axis 127 in the vicinity of MRAM storage element 100 isdisposed substantially parallel to longitudinal axis 129. A firstinsulating layer 130 is disposed below the first read conductor 128 andelectrically separates the read conductor 128 from a first writeconductor 132. The second MRAM storage element 200 is coupled at anupper surface to a second read/write conductor 226 and at a lowersurface to a second read conductor 228. The second read/right read/writeconductor 226 has a longitudinal axis 227 in the vicinity of the MRAMstorage element 100. The second read conductor 228 also has alongitudinal axis 229. Longitudinal axis 227 in the vicinity of MRAMstorage element 200 is also disposed substantially parallel tolongitudinal axis 229. A second insulating layer 230 is disposed belowthe speed read conductor 228 and separates it from a second writeconductor 232.

Insulating material 154 is disposed around the upper portion 120 of theMRAM cell 10. Likewise, insulating material 254 is disposed around thelower portion 160 of the MRAM cell 10. The insulating materials 154, 254may be a single material and may form a single body about both the upper120 and lower 160 portions of the MRAM cell 10, or may be formed as aplurality of layers of insulating material.

A via 152 passes through the insulating materials 154, 254 between thefirst read controller 128 and second read conductor 228. An electricalconductor 150 within the via 152 electrically couples the first 128 andthe second 228 read conductors.

Electric currents passed through the first read/write conductor 126 andfirst write conductor 132 can program the first MRAM storage element100. Likewise, electric currents passed through the second read/writeconductor 226 and second write conductor 232 can program the second MRAMstorage element 200. The first and second MRAM storage elements 100, 200can thus be independently programmed. The arrangement shown in FIG. 1Aalso allows the first and second MRAM storage elements 100, 200 to beindependently read by the conductors 126, 128 for memory element 100 andconductors 226, 228 for memory element 200.

The cell 10 also includes an access transistor 240 that allows MRAMstorage elements 100, 200 to be switchingly electrically connected to aground. Transistor 240 includes a first drain region 252 disposed withinthe doped well 354. Also within the doped well 354 is a second sourceregion 258 disposed in spaced relation to the first drain region 252.The source region 258 may be grounded through, for example, a metallicor polysilicon grounding conductor 259 to a remote source of groundpotential 263. A channel region 264 is defined in the doped well 354between the drain region 252 and source region 258. It should beunderstood that, transistor 240 may be an N-channel transistor or aP-channel transistor with corresponding source/drain conventions. Thus,the invention is not limited to a transistor 240 of a specific type.

The substrate 255 has an upper surface 266. A layer of gate insulatingmaterial 272 is formed over the upper surface 266 above the channelregion 264. A gate conductor 276 is formed over insulating layer 272. Inan exemplary embodiment, as shown in FIG. 1A, a landing pad 278 ofconductive material, e.g., doped polysilicon, is formed in contact withthe upper surface 266 of the substrate 255 above the drain region 252 toform an ohmic contact.

A via 353 passes through insulating material 254 between the landing pad278 and the second read conductor 228. A conductor 350 within the via352 electrically couples the second read conductor 228 to the landingpad 278, and thereby to the drain region 252. Accordingly, the firstread conductor 128 is coupled to the drain region 252 by way of thefirst conductor 150, second read conductor 228, second conductor 350 andlanding pad 278.

FIG. 1B shows another embodiment of the invention. Like the embodimentof FIG. 1A access transistor 240 includes a grounded source region 258.Unlike the FIG. 1A embodiment, the FIG. 1B embodiment shows the sourceregion 258 connected to the well 354 of doped semiconductor material.The doped well 354, in turn, is coupled to a source of ground potential263 and serves as a ground for source region 258. The source region 258may be coupled to the grounded well 354 by a conductor 261, such as ametallic or polysilicon conductor, and ohmic contacts.

Other access transistor structures may also be employed in place oftransistor 240. In a further embodiment, for example, a dual transistoras shown in FIG. 3A is used. The advantage of this arrangement is thatit is similar to a dual access transistor arrangement used in some FLASHmemory devices. Therefore, the dual transistor arrangement of FIG. 3Amay benefit from employing proven process technology. In addition, itmay be possible to use existing production facilities, previouslydeveloped for manufacturing FLASH memory devices, to manufacture devicesaccording to the present invention. The dual transistor has a secondsource region 256 disposed within the well 354 and coupled through asecond channel region 262 to the drain region 252. A second gate 274overlies the second channel above a second insulating layer 270. Gates276 and 274 are mutually electrically coupled by a conductor 275 andoperate together. Thus both transistors of the dual transistor structureare conductive simultaneously, and the two transistors of the dualtransistor structure act in parallel to switchingly ground the readconductors 128, 228.

FIG. 3B shows an arrangement of transistors such as might be used in aFLASH memory device. The array includes a plurality of conductors 275,transistor gates 274, 276, and regions 256, 258.

FIG. 4A shows a top view of a portion 120 of an MRAM cell 10 employed inthe previously described embodiments of the invention. As seen in FIG.4A the MRAM cell 10 includes portions of the conductors 126, 128, 132that couple the MRAM storage element 100 to the control and sensingcircuitry of the MRAM device. In the illustrated embodiment, the MRAMstorage element 100 has a substantially elliptical configuration, asviewed from above. Read/write conductor 126 is disposed above the memoryelement 100 in contact with the sense layer 110 (as detailed in FIGS. 1and 2). In like fashion, read conductor 128 is disposed below thestorage element 100 in contact with the seed layer 102 (as shown inFIGS. 1 and 2). Below the read conductor 128 is a layer of insulatingmaterial 130 (as shown in FIG. 1A) that separates the read conductor 128from a write conductor 132 that is disposed below the layer ofinsulating material 130. Also shown is the longitudinal axis 127 of thefirst read/rightread/write conductor 126 substantially parallel to thelongitudinal axis 129 of the first read conductor 128.

The read/write conductor 126 and the write conductor 132 are illustratedas being disposed in substantially perpendicular spaced relation to oneanother. In practice, the read/write conductor 126 and the writeconductor 132 may be disposed in oblique spaced relation to one another,so as to allow a vector sum of magnetic fields produced about theread/write and write conductors to locally exceed a magnetizationcoercivity threshold of the sense layer, whereby the sense layer isre-magnetized and programmed.

FIG. 4A also illustrates a conductor 350 passing through a via 352. Theconductor 350 is coupled at a first end to the sense layer of the MRAMstorage element 100 through the read conductor 128. The conductor 350 iscoupled at a second end to an access transistor 240. In the illustratedembodiment of FIG. 4A, the read conductor 128, magnetic memory element100 and read/write conductor 126 are surrounded by insulating material154. FIG. 4B illustrates an embodiment having stacked regionsportions120, 160 of for memory cells. As illustrated in FIG. 4B, regionportion120 is stacked above regionportion 160. RegionPortion 120 includes asingle MRAM storage element 100 and is substantially similar to FIG. 4A.However, regionportion 120 also includes a via 152 filled with aconductor 150. The via 152 and conductor 150 couple the memory cells120, 160elements 100, 200of the upper 120 and lower 160regionsportions.As shown in FIG. 4B, the lower regionportion 160 is substantiallysimilar to the upper regionportion 120, but includes its own read/writeconductor 226 and write conductor 232. It should be noted that only oneof the cellscell portions 120, 160 includes the via 352 and conductor350 which couples to the access transistor 240. In the exemplaryembodiment illustrated in FIG. 4B, only the bottom cell 160 includes via352 and conductor 350. Although FIG. 4B illustrates only tworegionsportions 120, 160, the principles of the present invention arealso applicable embodiments employing more than two verticalregionsportions. Further, while only one memory cell (e.g., cell withelement 100) in shown in each regionportion, in an actual memory deviceeach regionportion would include a large plurality of memory cells.

FIG. 5 shows a spatial relationship of memory cells 10 according to thevarious embodiments described above. Control transistors of a pluralityof memory cells 10 are disposed in a first two-dimensional layer 241which extends in a first direction 302 and a second direction 304. Thislayer includes the access transistors shown, for example, in FIGS. 1 and3A. Also shown are a first layer of memory cell upper portions 120 and asecond layer of memory cell lower portions 160. As shown, the layers ofthe upper and lower portions 120, 160 are stacked in a third direction306 while each of the first and second memory portion layers extendstwo-dimensionally in the first and second directions 302, 304. Together,the layers of upper and lower memory cell portions 120, 160 form athree-dimensional array of memory cell portions 308. As discussed above,the control transistors 240 may each include a single transistor, a dualtransistor, or another switching device. While FIG. 5 shows two layersof memory elements, additional memory element layers may also beprovided, with stacked memory elements being connected with the accesstransistors in the manner illustrated in FIGS. 1 and 3.

FIG. 6 shows the MRAM cell 10 described above with respect to FIG. 1A inelectrical schematic form. The cell 10 includes upper portion 120, lowerportion 160 and access transistor 240. The upper portion 120 includesthe first MRAM storage element 100 and first write conductor 132. Thelower portion 160 includes the second MRAM storage element 200 andsecond write conductor 232. The first read/write conductor 126 iscoupled to one end of the first MRAM storage element 100, and the secondread/write conductor 226 is coupled to a corresponding end of the secondMRAM storage element 200. The respective other ends of the MRAM storageelements 100, 200 are coupled to the drain D of the access transistor240. A word line conductor 320 is coupled to a gate G of the accesstransistor 240. The source S of the access transistor 240 is coupled toa source of constant potential such as ground 322.

FIG. 7 shows a portion of the MRAM memory device according to one aspectof the invention. The MRAM device includes a plurality of memory cells10, each having, for example, two resistive memory elements 130, 230.The resistive memory cells are each coupled to a respective read/writeconductor 126, 226. Each memory cell also includes an access transistor240. The access transistors are coupled at their gates to respectiveword line conductors 320 in a nor-structured architecture. Alsoillustrated is a path for a sense current 231 through a selectedresistive memory element. The sense current 231 flows from theread/write conductor 126, through the selected memory element and theaccess transistor 240 to ground 263. One leakage current path 233 isalso shown. The leakage current path shown 233 traverses a firstread/write line 126, a first resistive memory element 130, a secondresistive memory element 230, a second read/write line 226, and a thirdresistive memory element 230. The resulting sneak path resistance issignificantly larger in comparison to the sneak path resistance presentin a crosspoint architecture array. If R is the resistance value for aresistive memory element, the sneak path resistance according to oneembodiment of the invention is equal to ((n+1)/(n−1)). R/M, where n isthe number of word lines and M is the number of resistive memory elementin a typical cell 10. This contrasts with the sneak peak resistancewhich is R/(n−1) where n is the number of rows or columns. The highersneak path resistance, according to present invention, significantlyreduces the difficulty of sensing the resistance state of a sensedmemory element.

FIG. 8 shows a portion of an MRAM memory device 600 with a memory arrayincluding a plurality of MRAM cells 10 in accordance with the FIG. 1Aembodiment of the invention. A plurality of word lines 320 are showncoupled to respective gates 276 of the access transistors 240 of theMRAM cells 10. Read/write conductors 126, 226 are coupled to themagnetic storage elements 100, 200 of respective MRAM cells 10. Themagnetic storage elements 100, 200 are switchingly coupled to ground 322by their respective access transistors 240. Each read/write conductor126, 226 is coupled to a respective selection and sensing circuit 380.

The selection and sensing circuits 380 switchingly couple the read/writeconductors 126, 226 to sensing circuits that detect a resistive state ofthe storage elements 100, 200 and convey the sensed state to an outputport or pipeline circuit 382 of the MRAM device 600. Read/writeconductors 126, 226 that are not in use for sensing of a memory element100, 200 are switchingly decoupled by the selection and sensing circuits380 and allowed to float. The word lines 320 are coupled to respectiveoutputs of respective line driver circuits 384. The line driver circuits384 drive the respective gates 276 of access transistors 240 to avoltage that is alternately above or below a threshold voltage.Respective inputs of the line driver circuits 384 are electricallycoupled to respective outputs of an address decoder circuit 386.

The address decoder circuit 386 receives a word selection address at aninput 388 and responsively activates the appropriate line driver circuit384. Operation of the access transistors 240 is thus controlledaccording to a word selection address received at the input 388 of theaddress decoder 386. Each access transistor 240 having a gate coupled tothe active word line becomes conductive and electrically connects arespective a pair of MRAM storage elements 100, 200 to ground. Each MRAMstorage element 100, 200 in a selected row 400 of MRAM cells 10 that isassociated with the active word line 320 is available to be sensed. Theselection and sensing circuits 380 are controlled such that a voltage of(or current into) a first read/write conductor (e.g., 226) is sensedwhile a second read/write conductor (e.g., 126) is allowed to float.This condition exists for each MRAM cell 10 of the selected row 400.Each sensed voltage (or current) is reflected by the respective sensingcircuit 380 as a logical state and output to the pipeline circuit oroutput port 382. During a further read cycle, or portion of a readcycle, the selection and sensing circuits 380 are controlled to sensethe second read/write conductor, i.e., 126, is sensed while 226 isallowed to float.

FIG. 9 shows the MRAM 600 memory device of FIG. 8 with the addition ofwrite circuitry. The selection and sensing circuits 380 are illustratedby column selection transistors 410, 412 and sensing circuits 414. Thegates of the column selection transistors 410, 412 are controlled byrespective enabling lines 418, 416. Accordingly, when a respectiveenabling line 416 is logic low, a plurality of corresponding columnselection transistors 412 are conductive, whereby respective read/writeconductors 226 are effectively grounded.

The read/write conductors 126, 226 are each coupled to an output of arespective write driver circuit 420. Each write driver circuit 420 is,in turn, coupled at a respective input to a read/write (conductor) writeaddress decoder circuit 422. In response to a particular addressreceived at an address input 424 of the read/write conductor writeaddress decoder 422, a respective write driver circuit 420 sources afirst write current onto the respective read/write conductor (e.g.,226). During an overlapping time interval, a write line write addressdecoder 450 outputs a signal to one of a plurality of write line drivers452. The write line write address decoder 450 selects the particularwrite line driver 452 based on an input received at an address input 460of the decoder 450. The selected write line driver 452 sources a secondwrite current onto the respective write line (e.g., 232). The combinedeffect of the first and second write currents is to generate a localizedmagnetic field in the vicinity of a particular MRAM storage element 100and 200, (e.g., identified as 462 in FIG. 9 for one memory element 200).The localized magnetic field is sufficient to rotate the respectivemagnetic domains and consequently reverse the magnetic field of therespective sense layer 110 (as seen in FIG. 2) of the MRAM storageelement 200. Accordingly, the logical data state represented by the MRAMstorage element 200 is changed.

The various write drivers 420, 452 are each adapted to receive a signalcontrolling the direction of the current output by the respective driverso that either a “1” or a “0” may be written to a particular MRAMstorage element 100, 200 (i.e., the storage element 100, 200 may bewritten or erased). After writing is complete, the respective states ofthe enabling lines 416, 418 are restored to a “read mode” state suchthat transistors 412 become non-conductive and transistors 410 becomesconductive.

FIG. 10 illustrates an exemplary processing system 900 which may utilizethe memory device 600 of the present invention. The processing system900 includes one or more processors 901 coupled to a local bus 904. Amemory controller 902 and a primary bus bridge 903 are also coupled thelocal bus 904. The processing system 900 may include multiple memorycontrollers 902 and/or multiple primary bus bridges 903. The memorycontroller 902 and the primary bus bridge 903 may be integrated as asingle device 906.

The memory controller 902 is also coupled to one or more memory buses907. Each memory bus accepts memory components 908 which include atleast one resistive memory device, e.g., MRAM memory device, 600 of thepresent invention. The memory components 908 may be a memory card or amemory module. Examples of memory modules include single inline memorymodules (SIMMs) and dual inline memory modules (DIMMs). The memorycomponents 908 may include one or more additional devices 909. Forexample, in a SIMM or DIMM, the additional device 909 might be aconfiguration memory, such as a serial presence detect (SPD) memory. Thememory controller 902 may also be coupled to a cache memory 905. Thecache memory 905 may be the only cache memory in the processing system.Alternatively, other devices, for example, processors 901 may alsoinclude cache memories, which may form a cache hierarchy with cachememory 905. If the processing system 900 include peripherals orcontrollers which are bus masters or which support direct memory access(DMA), the memory controller 902 may implement a cache coherencyprotocol. If the memory controller 902 is coupled to a plurality ofmemory buses 907, each memory bus 907 may be operated in parallel, ordifferent address ranges may be mapped to different memory buses 907.

The primary bus bridge 903 is coupled to at least one peripheral bus910. Various devices, such as peripherals or additional bus bridges maybe coupled to the peripheral bus 910. These devices may include astorage controller 911, a miscellaneous I/O device 914, a secondary busbridge 915 communicating with a secondary bus 916, a multimediaprocessor 918, and a legacy device interface 920. The primary bus bridge903 may also coupled to one or more special purpose high speed ports922. In a personal computer, for example, the special purpose port mightbe the Accelerated Graphics Port (AGP), used to couple a highperformance video card to the processing system 900.

The storage controller 911 couples one or more storage device 913, via astorage bus 912, to the peripheral bus 910. For example, the storagecontroller 911 may be a SCSI controller and storage devices 913 may beSCSI discs. The I/O device 914 may be any sort of peripheral. Forexample, the I/O device 914 may be a local area network interface, suchas an Ethernet card. The secondary bus bridge 915 may be used tointerface additional devices via another bus to the processing system.For example, the secondary bus bridge may be a universal serial port(USB) controller used to couple USB devices 917 to the processing system900. The multimedia processor 918 may be a sound card, a video capturecard, or any other type of media interface, which may also be coupled toone or more additional devices such as speakers 919. The legacy deviceinterface 920 is used to couple at least one legacy device 921, forexample, older style keyboards and mice, to the processing system 900.

The processing system 900 illustrated in FIG. 10 is only an exemplaryprocessing system with which the invention may be used. While FIG. 9illustrates a processing architecture especially suitable for a generalpurpose computer, such as a personal computer or a workstation, itshould be recognized that well known modifications can be made toconFig. the processing system 900 to become more suitable for use in avariety of applications. For example, many electronic devices whichrequire processing may be implemented using a simpler architecture whichrelies on a CPU 901 coupled to memory components 908 and/or memorydevices. The modifications may include, for example, elimination ofunnecessary components, addition of specialized devices or circuits,and/or integration of a plurality of devices.

Although the invention has been illustrated as a memory cell 10 havingtwo independently controlled memory elements 100, 200 sharing a commonaccess transistor, the principles of the invention may be extended tothree or more independently controlled memory elements stacked in themanner of memory elements 100, 200. In any case, the result is athree-dimensional array of memory elements formed of layers of memoryelements stacked in first direction. Each transistor of atwo-dimensional array of transistors controls the memory elementsstacked above it in the three-dimensional array.

While exemplary embodiments of the invention have been described in theillustrations above, it should be understood that these are not to beconsidered as limiting. Although, for exemplary purposes, the discussionabove primarily covers devices with MRAM memory elements, devices withother memory elements, such as PCRAM memory elements, also fall withinthe invention. Addition, deletions, substitutions, and othermodifications can be made without departing from the spirit or scope ofthe present invention. Accordingly, the invention is not to beconsidered as limited by the foregoing description but is only limitedby the scope of the appended claims.

1. A magnetic random access memory cell comprising: a first magneticstorage element having a first sense layer and a first pinned layer; asecond magnetic storage element having a second sense layer and a secondpinned layer, said first and second sense layers being mutuallyelectrically coupled through first and second read conductors, saidfirst and second read conductors having respective longitudinal axes,said first and second pinned layers being electrically coupled torespective first and second read/write conductors, said first and secondread/write conductors having at least localized longitudinal axes inrespective vicinities of said first and second magnetic storageelements, said at least localized longitudinal axes of said firstread/write conductor being oriented substantially parallel to saidlongitudinal axis of said first read conductor; a switching devicecoupled to said mutually coupled pinned layers through said first andsecond read conductors and configured to couple said mutually coupledpinned layers to a conductor for receiving a substantially constantpotential.
 2. A magnetic random access memory cell as defined in claim 1wherein said substantially constant potential comprises a groundpotential.
 3. A magnetic random access memory cell as defined in claim 1wherein said first magnetic storage element and said second magneticstorage element are disposed above said switching device in a firstdirection, and wherein said switching device is disposed adjacent to asecond switching device of a second magnetic random access memory cell.4. A memory device comprising: a plurality of read/write conductorsrespectively paired with a plurality of read conductors, saidrespectively paired read/write and read conductors having substantiallyparallel longitudinal axes; and at least one memory cell electricallycoupled to each said respectively paired read/write conductor and readconductor, said at least one memory cell including a transistor and tworesistive memory elements, said two resistive memory elements beingelectrically connected in series by respective said read conductors,said two resistive memory elements being mutually coupled to saidtransistor at a common node.
 5. A memory device as defined in claim 4wherein: said two resistive memory elements each include a pinned layerand a sense layer; and wherein said sense layer of each said resistivememory element is electrically coupled through said transistor to aconductor for receiving a substantially constant electrical potential.6. A memory device as in claim 5 wherein said substantially constantelectrical potential is a ground potential.
 7. A memory device asdefined in claim 4 wherein said transistor comprises: two transistorshaving a common drain connection and respective gate terminals, saidgate terminals mutually coupled to one another.
 8. A memory device asdefined in claim 4 wherein: said first and second resistive memoryelements are disposed in layered spaced relation to one another abovesaid transistor.
 9. A memory device as defined in claim 4 furthercomprising: a word line conductor electrically coupled to a gate of saidtransistor.
 10. A memory integrated circuit comprising: a firsttwo-dimensional array of resistive memory elements disposed insubstantially parallel spaced relation between a second two-dimensionalarray of resistive memory elements and a third two-dimensional array ofisolation devices, each isolation device of said third two-dimensionalarray being coupled to at least one resistive memory element of saidfirst two-dimensional array and at least another resistive memoryelement of said second two-dimensional array; a first plurality ofread/write conductors having respective longitudinal axes oriented in afirst direction and coupled to said first two-dimensional array ofresistive memory elements; and a second plurality of read conductorshaving respective longitudinal axes also oriented in said firstdirection and also coupled to said first two-dimensional array ofresistive memory elements.
 11. A memory integrated circuit as defined inclaim 10 wherein said first array of resistive memory elements comprisesan array of MRAM memory elements.
 12. A memory integrated circuit asdefined in claim 10 wherein said first array of resistive memoryelements comprises an array of progrmamable conductive memory elements.13. A memory integrated circuit as defined in claim 10 furthercomprising: a sensing circuit, said sensing circuit adapted to sense astate of said resistive memory elements during a time interval when arespective isolation device is activated.
 14. A memory integratedcircuit comprising: a plurality of memory cells, each cell including:first and second resistive memory storage elements, said first andsecond resistive memory storage elements being electrically coupled torespective first and second memory sensing circuits, said first andsecond resistive memory storage elements being mutually coupled to areference potential through a common dual transistor.
 15. A memoryintegrated circuit as defined in claim 14 wherein said first and secondresistive memory storage elements are disposed in spaced relation abovesaid common dual transistor.
 16. A memory integrated circuit as definedin claim 14 further comprising an address decoder electrically coupledto first and second gates of said dual transistor and adapted toactivate said dual transistor in response to an address signal receivedat an address input of said address decoder.
 17. A magnetic randomaccess memory device comprising: a semiconductor substrate having anupper surface; a transistor having a drain region supported by saidsemiconductor substrate; a first magnetic random access memory storageelement over said upper surface and above said drain region andelectrically coupled to said drain region through a first readconductor, said first read conductor having a first longitudinal axis; asecond magnetic random access memory storage element over said uppersurface and above said first magnetic random access memory storageelement and electrically coupled to said first magnetic random accessmemory storage element and said drain region through a second readconductor, said second read conductor having a second longitudinal axis;and first and second read/write conductors having respective third andfourth longitudinal axes, said first longitudinal axis being disposedsubstantially parallel to said third longitudinal axis, said secondlongitudinal axis being disposed substantially parallel to said fourthlongitudinal axis.
 18. A programmable conductive memory devicecomprising: a semiconductor substrate having an upper surface; atransistor having a drain region supported by said semiconductorsubstrate; a first programmable conductive memory storage elementdisposed above said upper surface and electrically coupled to said drainregion by a first read conductor, said first read conductor having afirst longitudinal axis; a second programmable conductive memory storageelement disposed above said first programmable conductive memory storageelement and electrically coupled to said first programmable conductivememory storage element and said drain region through a second readconductor, said second read conductor having a second longitudinal axis;and first and second read/write conductors having respective third andfourth longitudinal axes, said first longitudinal axis being disposedsubstantially parallel to said third longitudinal axis, said secondlongitudinal axis being disposed substantially parallel to said fourthlongitudinal axis.
 19. A method of manufacturing forming a digital datastorage memory device, comprising: forming a transistor layer, includinga plurality of transistors, over a semiconductor substrate, saidtransistor layer comprising an array of transistors; forming a firstresistive memory storage layer over said transistor layer, said firstresistive memory storage layer comprising a plurality of first resistivememory storage structures elements, each of said plurality of firstresistive memory storage structures including respectively paired and aplurality of first read conductors and first read/write conductors, saidfirst read conductors and said first read/write conductors beingrespectively coupled with said plurality of first resistive memoryelements, wherein said first read conductors and said first read/writeconductors each have a respective first longitudinal axis and saidlongitudinal axes of said respectively paired read conductors andread/write conductors axes, which are disposed in a substantiallyparallel relationship ; forming a second magnetic memory storage layerover said first magnetic memory storage layer, said second magneticmemory storage layer comprising a plurality of second magnetic resistivememory storage structures elements, and a plurality of second readconductors and second read/write conductors, said second read conductorsand said second read/write conductors being respectively coupled withsaid plurality of first resistive memory elements, wherein said secondread conductors and said second read/write conductors each haverespective second longitudinal axes, which are substantially parallel tosaid first longitudinal axes; and electrically coupling respective onesof said plurality of said transistors, said plurality of first magneticmemory storage structures read conductors, and said plurality of secondmagnetic memory storage structures read conductors.
 20. A The method ofmanufacturing a digital data storage device as defined in claim 19,further comprising forming a control circuit over said semiconductorsubstrate, said control circuit being configured for activating saidplurality of transistors, said plurality of first resistive memoryelements, and said plurality of second resistive memory elements in athree-dimensional memory array comprising said transistor layer, saidfirst memory layer and said second memory layer.
 21. A processing systemcomprising: a plurality of memory cells, each cell including: first andsecond resistive memory storage elements, said first and secondresistive memory storage elements being electrically coupled torespective first and second memory sensing circuits, said first andsecond resistive memory storage elements being mutually coupled to areference potential through a wired-NOR FLASH memory transistor.
 22. Amethod of forming a memory device comprising: forming a plurality of NORFLASH-memory transistors disposed in an array over a semiconductorsubstrate; forming an array of first resistive memory elements over saidtransistors; forming an array of second resistive memory elements oversaid first resistive memory elements; and electrically coupling at leastone second resistive memory element to a respective first resistivememory element and to a respective transistor.
 23. A memory devicecomprising: a first magnetic storage element and a second magneticstorage element, wherein said first and second magnetic storage elementsare electrically coupled through associated respective first and secondread conductors and said first and second read conductors havesubstantially parallel respective first longitudinal axes; respectivefirst and second read/write conductors electrically coupled to saidfirst and second magnetic storage elements, said first and secondread/write conductors having substantially parallel respective secondlongitudinal axes, said first longitudinal axes are substantiallyparallel to said second longitudinal axes; respective first and secondwrite conductors associated with said first and second magnetic storageelements, wherein said first and second write conductors havesubstantially parallel third longitudinal axes which are non-parallelrelative to said first and second longitudinal axes; and a switchingdevice coupled to said first and second read conductors.
 24. The memorydevice of claim 23, wherein said switching device is configured tocouple said first and second read conductors to a constant potentialconductor.
 25. The memory device of claim 24, wherein said constantpotential conductor is a ground potential conductor.
 26. The memorydevice of claim 23, wherein said switching device comprises an accesstransistor.
 27. The memory device of claim 23, wherein said firstread/write conductor is over said first magnetic storage element,wherein said first magnetic storage element is over said first writeconductor, and wherein said first write conductor is over said firstwrite conductor.
 28. The memory device of claim 23, wherein said firstwrite conductor is over said second read/write conductor, wherein saidsecond read/write conductor is over said second magnetic storageelement, wherein said second magnetic storage element is over saidsecond read conductor, wherein said second read conductor is over saidsecond write conductor, wherein said second write conductor is over saidswitching device, and wherein said switching device is over a substrate.29. The memory device of claim 23, wherein said third longitudinal axesare at an oblique angle relative to said first and second longitudinalaxes.
 30. The memory device of claim 23, wherein said third longitudinalaxes are substantially orthogonal to said first and second longitudinalaxes.
 31. A memory device, comprising: an array of memory cells, eachsaid memory cell comprising at least two memory elements electricallycoupled to a common read circuit and therethrough to an accesstransistor, said access transistor being connected to one of a pluralityof wordlines, wherein each of said two memory elements is alsoelectrically coupled to a respective read/write line; and wherein asneak path resistance for reading one of said memory cells is determinedby the formula ((n+1 )/(n−1 ))·R/M, where R is the resistance value forone of said memory elements, n is the number of wordlines of said arrayand M is the number of memory elements in each of said memory cells. 32.The memory device of claim 31, wherein said access transistor is pairedwith a second access transistor in a NOR architecture.
 33. The memorydevice of claim 31, wherein each of said memory cells has exactly twomemory elements.
 34. The memory device of claim 31, wherein said memoryelements are MRAM memory elements.
 35. A method of reading a memorydevice comprising a memory cell comprising at least two memory elementselectrically coupled to one another by respective read lines and eachsaid memory element being connected to a respective read/write line,wherein said read lines of said memory cells are electrically coupled toa respective access transistor and a wordline respectively coupled to agate of said access transistor, said method comprising: electricallyconnecting said at least two memory elements to ground by activatingsaid wordline; selecting a read/write line associated with one of saidat least two memory elements; and sensing the resistivity state of thememory element associated with the selected read/write line as a logicstate while allowing the other of said at least two memory elements tofloat.
 36. The method of claim 35, wherein said sensing the resistivitydetects a voltage of the read/write line.
 37. The method of claim 35,wherein said sensing the resistivity detects a current into theread/write line.
 38. A method of writing a memory device comprising aplurality of memory cells, each comprising at least first and secondmemory elements electrically coupled to one another by respective readlines and each said memory element being connected to a respectiveread/write line and associated with a respective write line, whereinrespective said read lines of each said memory cell are electricallycoupled to a respective access transistor, said method comprising:addressing said first memory element by selecting a respectiveread/write line and write line; supplying a first write current to therespective read/write line; and supplying a second write current to therespective write line.
 39. The method of claim 38, wherein saidaddressing a memory element comprises activating a read/write line writeaddress decoder and a write line write address decoder.
 40. The methodof claim 38, wherein said supplying of first and second write currentsforms a localized magnetic field at said first memory element.
 41. Themethod of claim 38, further comprising restoring said first memoryelement to a read mode.